T651 ALLOY DURING RRA HEAT TREATMENT

Technical Field

The present invention relates to AA7075 alloys which have mainly mechanical features such as corrosion, hardness and strength etc.

State of the Art

In the state of the art, it is known that the strength increase is provided in AA7075 alloys after the RRA process applied after T6/T651 heat treatment. Heat treatments are widely used in the aluminum alloys so as to develop the stress corrosion crack sensitivity. As it is reported in the article titled "Structure-characteristic relations in A1 7050 and A1 7055 high-strength aluminum alloys", application of excessive aging in the industrial applications is a widespread method for T7xxx. Although alloys of 7000 series have excellent stress corrosion crack strength, their strength is 10-15% lower compared to the strength of T651-tempered alloys.

The aging heat treatment which is a strength-increasing heat treatment in aluminum alloys was first presented by A. Wilm in 1906 and patented under the name of "Aluminum Nobilitation" (DRP244554).

In the early 1970s, a method known as Retrogression and Re- Aging (RRA) registered under US Patent was suggested by B.Cina so as to increase both the strength and the stress corrosion crack strength in 7XXX aluminum alloys registered with US Patent 3856584. Aging process (T651) that is also known as precipitation hardening can be applicable to the alloys at use temperature in general, when it is in at least two-phase structure with thermodynamic balance, that can be transformed into a single phase semi-stable (supersaturated solid melt) structure by not permitting the formation of balance structure phases by shock cooling. The high strength of 7000 series alloys depends on the fine and homogenously dispersed precipitates in the precipitated matrix during artificial aging. The normal precipitation sequence of 7000 series Al alloys was summarized as follows in the "Early stage precipitation in Al - Zn - Mg - Cu alloy" study performed by Sha, G. in 2004: solid precipitate - GP region - metastable h -stable h (MgZn2).

Some studies have shown that aging behavior of an A1 7075 alloy is significantly influenced by pre-deformation before the aging heat treatment. In practice, Kagar and Guleryuz has examined that aging duration and temperature, quenching rate and prestress have a critical effect on the strength of A1 7075, in their studies on "The effect of quenching rate and pre-stress on the stress aging behavior of 7075 aluminum alloys" .

In the study titled "The effect of pretensioning on the aging of A1 7075 alloy with hot pressing feature" performed in 2018 by Seon-Ho Jung et al., pre-deformation was applied to the supersaturated solid solution with a tensile stress of 5-10% and the tensile properties were tried to be developed, during the dissolution phase of the T6 aging heat treatment and after quenching .

Technical problems aimed to be solved with the Invention The inventive thermo-mechanically treated aluminum alloys are aimed to have high strength due to both RRA heat treatment applied after precipitation hardening (T651) and pre deformation applied together with the temperature during RRA. They are expected to have high tensile strength with ductility and increased strength. It is aimed to use as engine parts, body components in the automotive industry due to its superior tensile strength.

Description of the Invention

AA7075 alloy that was commercially purchased and was subjected to T651 heat treatment was cut into ASTM-E8 tensile test samples by laser cutting method in this study. A pre deformation was applied to the test samples obtained as tensile samples in a ratio of 5% and 10% during RRA heat treatment after T651 heat treatment. In this method, the first step of RRA heat treatment, in the retrogression process, it is kept at 200 °C for 10 minutes with the resistance in the tensile test device and is shock cooled by deforming the same by 5-10% at this temperature and re-aging process is applied at 120°C for 24 hours.

It is shown in graphs in the attached figure and the explanation is as follows;

Figure 1- Summary of the Thermo-mechanical Process Applied

Industrial Applicability of the Invention

AA705+ T651 aluminum alloy that was pre-deformed during RRA process can be used in the production of passenger compartments in the aircraft in aviation industry, in the production of ship bodies and propellers in the ship building, in the production of radiators, motor components, body sheet plates, structural parts in the automotive industry.

AA7075+T651 alloy has the potential to be used in the construction sector in aesthetic applications and in food industry, in tank and pipes that require corrosion resistance by optimizing the predeformation amount applied during RRA and/or optimizing the durations.